Harare - While cash-strapped Zimbabwe is struggling to pay its civil servants, struggling state enterprises and government departments are being asked to pay for advertising space ahead of President Robert Mugabe's wife Grace's birthday this weekend.She turns 51 on Saturday. The former secretary is more than 40 years younger than her husband and has denied persistent rumours that she is angling to succeed him.Calling the first lady "unconquerable", the official Herald newspaper said it would publish a "very special supplement" to mark her birthday. It asked government departments and parastatals to buy advertising space to pay for it.In a small insert, also carried by other government-controlled papers at the weekend, the Herald said: "The supplement will carry all the good work that Amai is doing in assisting the disadvantaged members of our society as part of the first lady's day to day work. "As a proud Zimbabwean, the first lady, Dr Amai Grace Mugabe, has made it a tradition to donate to charities," it continued.Amai, or mother, is a popular name for the Zimbabwean first lady. A lavish dinner dance was held to mark Grace Mugabe's 50th birthday in Harare last year.It's not clear what form celebrations would take this year, to be held against the background of growing challenges to Mugabe's long-time hold on power. The first lady - whose reputation as a keen overseas shopper was cemented during Zimbabwe's last economic crisis up to 2008 - reportedly flew out of Harare last week ahead of a shutdown called by the #ThisFlag movement. She apparently travelled to Singapore, where the couple's only daughter, Bona, had a baby in April. State media carried a glowing tribute to Grace Mugabe this week. Mugabe's chief secretary Misheck Sibanda said she was a "selfless philanthropist”, an "outstanding entrepreneur" and a "bastion for the defence of good moral and family values".Civil servants in Zimbabwe would get their salaries late this month. Authorities were expected to announce pay dates on Friday. In a blog post this week, top opposition official Eddie Cross warned the cash squeeze would reach "critical levels" in August.